Method of stop-off of cathode pieces in chromium plating baths



Dec. 10, 1968 R 55m ET AL 3,415,723

METHOD OF STOP-OFF OF CATHODE PIECES IN CHROMIUM PLATING BATHS Filed July 1. 1963 United States Patent 3,415,723 METHOD OF STOP-OFF OF CATHODE PIECES IN CHROMIUM PLATING BATHS Ram Dev Bedi, Oak Park, and George Dubpernell, Huntington Woods, Mich., assignors to M & T Chemicals Inc., New York, N.Y., a corporation of Delaware Filed July 1, 1963, Ser. No. 291,988 9 Claims. (Cl. 204--) ABSTRACT OF THE DISCLOSURE A process for electroplating a chromium plate from a chromium plating bath onto a workpiece having predetermined areas to be chromium plated and selected areas to be stopped-off which comprises placing a piece of etched high'carbon iron alloy immediately adjacent to and in electrical contact with said selected areas, subjecting said selected areas to plating current density whereby said plate is deposited onto the predetermined areas while the selected areas remain stopped-off, and to chromium plated products produced by said process.

This invention relates to a method of treating cathodes. More particularly, this invention relates to a method of treating selected areas of cathodes to effect stopping-off, he to permit control of selected areas of a cathode used e.g. in a chromium plating bath so that they may be free from plate which is to be deposited on other areas of the cathode.

As is well known to those skilled in the art, metals may be immersed in various baths in the course of different treating processes wherein desired portions of the metal may, for example, be subjected to electroplating. As is well known to those skilled in the art, electroplating of various metals, typically nickel, copper, tin, zinc, or chromium may be effected in baths of varying acidity and composition; many of these baths may contain chloride or fluoride which may be an integral part of the bath or present as an additive. Because of differences in area or geometry or accessibility, there may be predetermined portions of the cathode which are to be plated and selected areas on which a plate is not desired. Specifically, the surface of these selected areas, if not stoppedolf (i.e. if not properly treated to prevent chromium plating thereon) may undesirably be plated sufficiently to alter the dimensions substantially and/or to spoil the appearance of the finished article. The problem of stopoff may be present in various baths, typified by chromium plating baths, nickel plating baths, acid copper baths, acid tin baths, zinc baths, etc.; for purpose of convenience, reefrence will hereinafter be made to chromium plating baths.

Commonly the areas of the cathode on which plate is not desired may be covered or painted with a masking or stop-off material which prevents deposition and build-up of the plate on these high current density areas which would normally be plated. The masking or stop-off coatings are commonly subjected to severe corrosive conditions for an extended period of time. For example, in chromium plating these materials may remain in position for extended periods of time, often for days, under the most severe oxidizing conditions in contact with baths which may contain sulfuric acid and chromic acid together wth numerous other chemicals. Furthermore the stop-olf material must be resistant to the highly erosive effect of the hydrogen which is liberated.

As is well known to those skilled in the art, chromium plating for example may be effected by use of a bath containing chromic acid and sulfate together with other compounds which may be employed to effect various de- 3,415,723 Patented Dec. 10, 1968 sirable results. Typical of these additive compounds may be fluoride or fluoride complexes. Other illustrative chromium plating systems may include soluble catalyst systems containing e.g. chromic acid and sulfate such as sulfuric acid in amount sufiicient to give the desired concentration of sulfate ion, or self-regulating baths, typically those containing silicofluorides together with sulfate.

'During chromium plating from baths typified by the foregoing, it is common to operate at a temperature which may vary depending upon the type and purpose of the plating operation. Commonly however the temperature of operation may be 34 C.-72 C. and typically 48 C.- 63 C. The cathodic current density may preferably be controlled to fall in the range of 8-100, and typically 12-45 a.s.d. (i.e. amperes per square decimeter) on the selected areas of high current density whereon plating may occur. However, because there may be certain selected high current density areas which are not to be chromium plated, it is desired to stop-off these areas.

In chromium plating systems, it has heretofore been common to coat with tapes or waxes those selected portions of the cathode where no plate is desired. The use of such techniques is time consuming and expensive. Tapes or waxes must be carefully placed and removed, since any holes or spaces (including edge areas which may be undermined) in the protective covering will allow plating of the so-exposed metal to take place. Such tapes or waxes may be expensive to purchase and apply. Various other attempts to stop-off selected areas of cathodes in the noted plating baths have not been uniformly successful, and there is today no economical stop-off system which is completely satisfactory.

Various other processes are also known, including chemical treating, etching of selected portions of a piece, anodizing of pieces, etc. wherein it is desired to protect certain portions of the workpiece from the action of the solution. Stop-off materials to effect these results have heretofore been either paints or tapes. Typical paints may include plastic materials such as polymers of vinyl chloride or vinyl acetate which may contain plasticizers, pigments, solvents, etc. These may be applied by painting onto the desired portions of the workpiece which are not to be subjected to the action of the solution. Masking tapes which may be employed for the same purpose may be wrapped around the piece and cover the areas which are to be protected or stopped-off.

As is well known to those skilled in the art, application of, e.g., masking tapes and paints is a time consuming and expensive job. Furthermore, many of the stop-off paints or tapes are unsatisfactory in that, during plating, they may become pervious to the solution or not adherent to the workpiece. Previousness permits growth of dendritic trees at or adjacent to the tape or paint; such growth must be removed by grinding or machining to make the part satisfactory with respect to appearance and dimensionality. It is a particular objection that tapes and paints permit build-up of the plate at the point immediately adjacent thereto and thus may require a grinding operation, after plating is complete, to remove the ridges which may be present. It is a further objection that paint or masking tape must be removed from the workpiece at the completion of the plating operation. This may typically be done by cutting, peeling, scraping, by action of solvents, etc.

It is an object of this invention to provide a novel technique for masking or stopping-off areas to be maintained free of a deposited plate during plating. Other objects will be apparent to those skilled in the art on inspection of the following description.

In accordance with certain of its aspects, the process of this invention for electroplating a plate metal from a bath onto a cathode having predetermined areas to be plated and selected areas to be stopped-off comprises protecting said selected areas with a piece of etched high-carbon iron alloy, and subjecting said cathode including said selected areas to plating current density whereby said plate metal is deposited on to said predetermined areas and said selected areas remain stopped-off.

Typical of the plate metal with which the process of this invention may find use may be the aforementioned chromium plate including hard chromium plate. This invention will be especially suitable for use with the noted chromium plating baths, including the soluble or sparingly soluble catalyst-containing chromium plating baths.

The cathodes which may be used in the practice of this invention may be those basis metal articles upon which a plate, typically a chromium plate is desired. Typically these basis metals may be characterized by their high hydrogen overvoltage. Hydrogen overvoltage is a measure of the amount of work required to liberate hydrogen at the cathode. Thus, it is conventionally expressed as the difference between the potential of the electrode when hydrogen is liberated during electrolysis and the potential of the reversible hydrogen electrode, both potentials being referred to the same electrolyte. A cathode having a lower hydrogen overvoltage will, therefore, generate hydrogen at a lower voltage than a cathode having a higher hydrogen overvoltage. High hydrogen overvoltage basis metals may include ferrous metals such as iron or steel, including stainless steels, low carbon steels, nickel steels, chromium steels, chromium-nickel steels, etc., particularly when these metals are in bright, solid, highly polished condition.

In practice of this invention, the selected areas of the cathode on which it is not desired that plating occur may be protected with a piece of etched high-carbon iron alloy. The high-carbon iron alloys which may be employed may be preferably selected from the group consisting of cast iron and high-carbon steels. Cast iron may be iron containing l.76% carbon which is not malleable over a wide range of temperatures. It may normally exist in the form of white cast iron and gray cast iron, the formerly apparently containing a chemical compound or alloy of iron and carbon, the latter containing a substantial portion of a mixture of the two materials. Although cast iron may contain other materials such as phosphorus, sulfur, silicon, copper, manganese, nickel, tungsten, etc., it will not normally have any substantial amount of added alloying metals. A typical cast iron may be one containing 4.2% combined carbon and 0.2% graphite and melting at 1088 C.

The high carbon steels which may be used in practice of this invention may include steel containing 0.60%- 0.90% carbon together with other optional preferred alloying metals. Typically these alloying metals may include manganese, chromium, nickel, and molybdenum. A typical high carbon steel which may be employed may be SAE Steel No. 1090 containing Percent Carbon 0.850.98

Manganese 0.60-0.90 Phosphorus, maximum 0.04 Sulfur, maximum 0.05

The high-carbon iron alloy piece may be etched by immersion for 1-10 seconds or longer, say seconds in mineral acid, e.g., hydrochloric acid, sulfuric acid, etc. at C.30 0, say 25 C. During this immersion the surface of the piece may become etched as hydrogen is liberated. The etched surface of the high-carbon iron alloy may be in low hydrogen overvoltage form commonly characterized by finely divided surface condition.

The high-carbon iron alloy may be used to protect those selected areas of the surface to be stopped-off. It may be employed to protect the cleaner or prepared surface of the basis metal in a number of ways. One highly convenient method for placing the high-carbon iron alloy in position to protect the selected areas of the cathode may be by use of a thin foraminous sheet, including mesh, expanded metal, perforated metal, etc. This may be in intimate electrical contact with, and preferably positioned immediately adjacent to and electrically connected to, the cathode areas to be stopped-off. The portion of the cathode to be stopped-off may preferably be overlaid with and contiguous to the thin foraminous sheet. Use of such a foraminous sheet immediately adjacent to and electrically connected to the cathode permits plating predetermined areas of the cathode while satisfactorily stopping-off selected areas. Preferably the etched high-carbon iron alloy will substantially envelope, surround, or isolate the selected areas.

In the preferred embodiment, the piece of etched highcarbon iron alloy which may be employed in practice of this invention may be a jig or fixture in which or by means of which the piece to be plated is mounted in the rack. Where the portion of the piece to be stopped-off may be, e.g., the threads on a bolt, the threads may be protected by use of a nut threaded thereover which covers the threads. The nut may be formed of a high-carbon iron alloy the surface of which has been etched as hereinbefore noted supra.

Because of the unexpectedly low hydrogen overvoltage on the surface of the etched high-carbon iron alloy, it may be possible to position preformed and preshaped pieces of said iron-carbon alloy at the extremities of areas to be stopped-01f (thereby delineating the areas to be plated) whereby the selected areas to be stopped-off are not exposed to the potential required to effect plating thereon.

It is a feature of this invention that the high-carbon iron alloy may be etched in the plating bath prior to electroplating. Where the bath e.g. a chromium plating bath is sufficiently corrosive to the particular high-carbon iron alloy, the assembly including this alloy may be permitted to remain in the bath for 30-120, say 60 seconds prior to electroplating. It may be found that this will etch the surface sufficiently to permit stop-off.

It is also a feature of this invention that etching may be effected by making the assembly including the highcarbon alloy anode for 560 seconds, say 10 seconds prior to plating e. g. with chromium.

Use of the particular alloys as stop-01f materials is highly desirable. The alloy may retain its activity indefinitely and it may not be readily poisoned or deactivated during normal operation of baths including chromium plating baths. In fact, the slightest corrosion or pickling action of the bath on the high carbon iron alloy serves to further activate the stop-off ability of the alloy. It is economical to use, in that it may be employed over and over again an almost unlimited number of times, and is conveniently removed after plating.

The following illustrative examples clearly show the novel features of the process of this invention:

Example 1 In this example, pieces 10, 11, and 12 of high-carbon iron alloy (having dimensions as hereinafter described) fabricated of plain cast iron were etched by immersion in hydrochloric acid for 10 seconds. They were then washed, dried and assembled in manner shown in FIG. 1. As is apparent, the pieces 10 and 11 may be fitting pieces of cast iron wherein the portion 14 of bolt 13 fits; the piece 12 in this embodiment may be a nut which fits over most of the threads 15 on bolt 13. The pieces 10 and 11 may be secured together by means not shown.

The entire assembly may be placed as cathode within a chromium plating bath containing 250 g./l. chromic acid, 1.25 g./l. sulfate ion as sulfuric acid, and 2.5 g./l. of silicofluoride ion SiF Power was supplied to maintain an effective cathode current density of 50 a.s.d. at C. for 5 hours. During this time, a plate 17 of 250 microns thickness was deposited on the predetermined area 16. The ends 18 .and 19 of the plate stopped at the begininng of the selected area 14 and the thread 15 of the bolt 13. These selected areas were free of plate. It was apparent that the novel technique of this invention permitted electroplating on the predetermined areas while simultaneously stoppingotl selected areas.

Example 2 In this example the procedure of Example 1 was duplicated except that the pieces 10, 11, and 12 of cast iron were not etched prior to immersion in the chromium plating bath. Etching of these pieces was effected by allowing the assembly containing these pieces to rest in the bath, without current for 60 seconds prior to plating.

It was found that the stopping-off elfect was attained as in Example 1.

Example 3 In this example, the procedure of Example 1 was duplicated except that the pieces 10, 11, and 12 of cast iron were not etched prior to immersion in the chromium plating bath. Etching of the pieces was eifected by making the assembly including pieces 10, 11, and 12 anode for 510 seconds.

It was found that the stopping-off effect was attained as in Example 1.

It 'will be apparent to those skilled in the art that a plated product prepared according to the process of this invention is unexpectedly novel in that it is characterized by a layer of plate of predetermined thickness which plate in its deposited form possesses smoothly delineated edges which gradually decrease in thickness over the transition area between the plate and the unplated metal base whereby the plate gradually blends into the metal base. This plated piece need not be ground to remove growths etc. at the end of the plated area as is common with pieces plated by standard e.g. chromium plating techniques.

While it has long been hypothesized that this would be desirable or that it could be done, it has not heretofore been possible to attain this desiderata. The ends of prior art plate, because of the stop-otf techniques, have always been at least as thick as and commonly thicker than the layer of plate. This is because of the particular effect of the prior art non-conducting stop-off materials on the flow of current at the portion of plate immediately adjacent thereto.

It will readily be appreciated that the novel technique of this invention for stopping-off cathodes by protecting selected areas thereon with high carbon iron alloys permits .attainment of superior results. These high carbon iron alloys which, when protecting i.e. when in intimate electrical contact with the cathode, permit attainment of this unexpected and superior result, may be readily fabricated in various forms. Although most commonly, as shown in the drawing, the high carbon iron alloys may be in the form of or a portion of the cathode-holding mechanism, it may be possible to use therein foraminous sheets including mesh, expanded metal, and perforated metal, which preferably may contain a plurality of holes of very small size, typically less than about 1 mm. in diameter.

As many embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention includes all such modifications and variations as come within the scope of the appended claims.

We claim:

1. The process for electroplating a chromium plate from a chromium plating bath onto a workpiece having predetermined areas to be plated and selected areas to be stopped-oil? which comprises placing a piece of etched high-carbon iron alloy immediately adjacent to and in electrical contact with said selected areas, and subjecting said selected areas to plating current density whereby said plate is deposited onto said predetermined areas and said selected areas remain stopped-off.

2. The process for electroplating a chromium plate from a chromium plating bath onto a workpiece having predetermined areas to be plated and selected areas to be stopped-off as claimed in claim 1 wherein said highcarbon iron alloy is cast iron.

3. The process for electroplating a chromuim plate from a chromium plating bath onto a workpiece having predetermined areas to be plated and selected areas to be stopped-01f as claimed in claim 1 wherein said highcarbon iron alloy is high-carbon steel.

4. The process for electroplating a chromium plate from a chromium plating bath onto a workpiece having predetermined areas to be plated and selected areas to be stopped otf as claimed in claim 1 wherein said highcarbon iron alloy is high-carbon steel containing 0.60%- 0.90% carbon.

5. The process for electroplating a. chromium plate from a chromium plating bath onto a workpiece having predetermined areas to be plated and selected areas to be stopped-01f which comprises placing a piece of etched high-carbon iron alloy immediately adjacent to and in electrical contact with said selected areas, and subjecting said workpiece including said selected. areas to plating current density whereby said plate is deposited onto said predetermined areas and said selected areas remain stopped-off.

6. The process for electroplating a chromium plate from a chromium plating bath onto a workpiece having predetermined areas to be plated and selected areas to be stopped-off which comprises placing a piece of highcarbon iron alloy immediately adjacent to and in electrical contact with said selected areas, placing said workpiece and said iron alloy in said bath whereby said iron alloy becomes etched, and subjecting said workpiece including said selected areas to plating current density whereby said plate is deposited onto said predetermined areas and said selected areas remain stopped-off.

7. The process for electroplating a chromium plate from a chromium plating bath onto a workpiece having predetermined areas to be plated and selected areas to be stopped-off which comprises placing a piece of highcarbon iron alloy immediately adjacent to and in electrical contact with said selected areas, placing said workpiece and said iron alloy in said bath, making said workpiece anode for time sutficient to etch said iron al- 10y, then maintaining said workpiece as cathode in said bath, and subjecting said workpiece including said selected areas to plating current density whereby said plate is deposited onto said predetermined areas and said selected areas remain stopped-off.

8. The process for electroplating a chromium plate from a chromium plating bath onto a workpiece having predetermined areas to be plated and selected areas to be stopped-01f as claimed in claim 7 wherein said workpiece is maintained as anode for 5-60 seconds thereby etching said workpiece prior to electroplating.

9. A novel product comprising a metal base and a layer of chromium metal plate on portions thereof, said layer of metal plate in its deposited form being characterized by smoothly delineated edges which gradually decrease in thickness over the transition areas between the plate portions and the unplated metal base.

References Cited UNITED STATES PATENTS 644,029 2/1900 Cowper-Coles. 1,750,418 3/ 1930 McFarland 20418 2,422,242 6/ 1947 Kaufman 204-15 2,784,151 3/ 1957 Topelian 204-279 3,014,851 12/1961 Bommerscheim 204- 15 FOREIGN PATENTS 25,393 1913 Great Britain.

HOWARD S. WILLIAMS, Primary Examiner. T. TUFARIELLO, Assistant Examiner. 

